The present invention relates to plasma vacuum treatment (etching or deposition) of a substrate.
Plasma vacuum substrate treatment processes are already known in the art, such as the etching process described in document WO 94/14187, in which the substrate is placed in a vacuum enclosure containing active gases injected in particular via controlled active gas injection means and evacuated via controlled gas aspiration means. The active gases are ionized by plasma-generating means. The substrate in the vacuum enclosure receives bombardment electrical energy generated by a controlled bombardment generator to which it is connected.
During an etching step, ions of a reactive material such as sulfur hexafluoride, generated by ionization, are accelerated by the bombardment energy and impinge on the substrate, for example a silicon-based substrate, in areas that are not covered by a mask. The active ions combine with the material of the substrate to form a gaseous substance that is evacuated by the gas aspiration means.
One problem is to obtain etching with well-defined outlines, i.e. in which the cavities have side walls that are as regular as possible and perpendicular to the surface of the substrate.
To this end, document WO 94/14187 teaches that etching should be carried out by means of an alternating succession of etching steps and protection layer generating steps. During a protection layer generating step, the vacuum enclosure contains an active gas of a first type, such as trifluoromethane (CHF3), which produces a polymerized layer over all of the surface of the substrate, including the bottom and side faces of the cavity. During an etching step the vacuum enclosure contains an active gas of a second type, such as sulfur hexafluoride, which produces ions which are accelerated towards the substrate to produce directional bombardment that impinges selectively on the bottom of the cavities to remove the material of the substrate selectively from the bottom of the cavities without attacking the side faces.
However, it is clear that the etching step lowers the bottom of the cavity relative to the bottom edge of the lateral polymerized protection layer, with the result that an area appears at the bottom of the side face that is not protected by a layer of polymerized material. The active ion bombardment tends to undercut the cavity at the bottom and widen it, affecting the regularity of the side face of the cavity.
To avoid this phenomenon, it is necessary to use a fast alternation of short etching and protection layer generating steps.
Another problem is to optimize performance in terms of etching and protection layer generation.
In most plasma vacuum treatment processes (etching or deposition), the main parameters that can be altered to optimize etching or deposition performance are: the flowrates of the various gases, the working pressure, the electromagnetic power coupled to the plasma to generate it, and the energy with which the substrate is bombarded.
Like the rate of deposition or etching, the selectivity relative to the mask, the regularity of the etching profiles, and the conformity of the deposited layers, etching or deposition performance depends on the conditions of the gaseous phase discharge in terms of the power coupled to the plasma, working pressure and substrate bombardment energy.
As a general rule, to optimize a deposition or etching process, the flowrates of the various gases, the electromagnetic power coupled to the plasma and the substrate bombardment energy are optimized at precise and constant values throughout the treatment.
For example, if patterns are to be plasma etched into a silicon wafer, the active gas flowrates are set at well-defined and constant values, the electromagnetic power coupled to the plasma to a precise value, and likewise the value of the working pressure and the bias voltage, which determine the energy with which the substrate is bombarded by the ions. The above values are held constant throughout the treatment step. In particular, the gas flowrates are set to a precise value and the pressure is maintained at a predetermined value using a pressure regulator system. The pressure regulator system is generally a pump system including at least one pump downstream of a regulator valve slaved to the working pressure, thereby constituting controlled gas aspiration means. Accordingly, regardless of the gas flowrates required, the pressure regulator valve is set so that it procures the required working pressure in the reaction chamber.
The above controlled gas aspiration means operate correctly to regulate the pressure in the event of slow variations in the gas flowrates. However, it is apparent that they cannot efficiently track fast variations in the gas flowrates, such as the variations that can occur if it is required to optimize not only the etching steps but also the protection layer generation steps in an etching process according to document WO 94/14187 by alternating the etching and protection layer generation steps at a sufficiently high frequency to attain sufficiently regular lateral cavity faces. For example, it may be beneficial for the alternating phases to include etching phases with a duration of approximately three seconds and protection layer generation phases with a duration of approximately one second. If the gas flowrate during the etching step is different from the gas flowrate during the protection layer generation step, the mechanical inertia of the controlled regulation valve is unable to track variations this fast.
Nor does the mechanical inertia of the controlled regulator valve allow rapid setting of the gas pressure to two successive different values to optimize two successive short treatment steps.
Another problem is that if the gas pressure inside the vacuum enclosure varies between two successive steps, this produces a variation in the impedance of the plasma as seen by the controlled bombardment energy generator connected to the substrate, and it is then no longer possible to control the bombardment energy efficiently to assure the regularity of the treatment.
The controlled bombardment energy generator is generally associated with an impedance adapter for matching the impedance of the energy source to that of the plasma. The impedance adapter can track slow variations in the impedance of the plasma, for which purpose it comprises a variable capacitor whose capacitance is varied by mechanically moving the capacitor plates relative to each other. However, a device of the above kind is not able to track fast impedance variations, i.e. variations occurring at a frequency in the order of 1 Hz.
The problem addressed by the present invention is therefore that of devising a process and a system in which the flow of active gas in the vacuum enclosure can be varied quickly for vacuum treatment of a substrate without interfering with the means for controlling and regulating ionic bombardment of the substrate.
Another object of the invention is to provide means for obtaining a controlled pressure in the vacuum enclosure using conventional controlled gas aspiration means employing pressure regulator valves but allowing the active gas flowrates to be varied quickly.
To achieve the above and other objects, in a plasma vacuum substrate treatment process of the invention the substrate is placed in a vacuum enclosure containing active gases injected via controlled active gas injection means and evacuated via controlled gas aspiration means, the active gases are ionized by plasma-generating means, and the substrate receives electrical bombardment energy generated by a controlled bombardment energy generator; the process includes at least one sequence of variation of the flow of injected active gas at a rate of variation greater than the capacity to regulate the gas pressure in the vacuum enclosure of the controlled gas aspiration means; the impedance of the plasma as seen by the controlled bombardment energy generator is maintained at a substantially constant level by auxiliary compensation means during the active gas injection flow variation sequence.
In an advantageous embodiment of the invention the impedance of the plasma as seen by the controlled bombardment energy generator is maintained substantially constant by controlled injection of at least one passive control gas into the vacuum enclosure to maintain substantially constant the total flow of gas injected into the vacuum enclosure.
The passive control gas is preferably injected into the vacuum enclosure in an area close to the control gas aspiration means in order to prevent it being ionized.
In a first application, the process includes a succession of steps of injecting active gas at a first injection flowrate and steps of injecting active gas at a second injection flowrate different from the first injection flowrate, at least some of the steps being shorter than the reaction time of the controlled gas aspiration means.
The bombardment power communicated to the substrate can differ from one step to another.
The process can be applied to etching a substrate, for example a silicon-based substrate. In this case, a succession of alternating short steps is advantageously used comprising an etching step in which the first active gas is SF6 during a first time period at a first flowrate and a protection layer generating step during which the second active gas is C4F8 during a second time period at a second flowrate with simultaneous injection of a complementary flowrate of a passive control gas such as nitrogen or argon.
The process can instead be applied to depositing a deposit on the substrate.
A plasma vacuum substrate treatment system of the invention for implementing a process of the above kind comprises:
a vacuum enclosure,
controlled active gas injection means for injecting gas into the vacuum enclosure,
controlled gas aspiration means for aspirating gas out of the vacuum enclosure,
plasma-generating means in the vacuum enclosure,
substrate support means for supporting the substrate in the vacuum enclosure,
a controlled bombardment energy generator adapted to communicate to the substrate placed on the substrate support means an appropriate level of bombardment energy, and
auxiliary compensation means for maintaining substantially constant the impedance of the plasma as seen by the controlled bombardment energy generator in the event of variation of the gas entry flowrate and for maintaining substantially constant the gas aspiration flowrate.
In an advantageous embodiment the auxiliary compensation means include means for controlled injection of at least one passive control gas into the vacuum enclosure.
The controlled control gas injection means are preferably adapted to inject the passive control gas into an area of the vacuum enclosure near the controlled gas aspiration means.
The system advantageously includes control means for controlling the controlled control gas injection means to maintain substantially constant the total flow of gas injected into the vacuum enclosure.